Modern gas turbine engines and their respective components, operate at high rotational speeds and high temperatures for increased performance and efficiency. Thus, the materials from which these components are made must be able to withstand severe operating environments.
Most high temperature gas turbine components are made of nickel base superalloys, which are alloys that are specifically developed for applications involving extreme temperatures and mechanical stresses. Superalloys are often cast, by an appropriate process, into the component shape. For example, directional solidification is known in the art. This casting technique aligns grain boundaries parallel to the stress axis. This alignment enhances elevated temperature strength by increasing resistance to creep and minimizing grain boundary failure initiation sites.
An extension of the above-described technique is single crystal casting. Casting of alloys in single crystal form eliminates internal crystal boundaries in the finished article. Single crystal turbine blades and vanes possess superior characteristics, such as strength, ductility and crack resistance at high operating temperatures. Thus, single crystal articles are extensively used in components of gas turbine engines.
Although single crystal engine components are desirable, they are extremely costly to manufacture. Defects often occur during manufacturing, as well as after extensive engine operation. Upon detection of certain critical defects, such as cracks, the component must be repaired, replaced or otherwise scrapped. This incurs a significant expense and is undesirable.
The fabrication of gas turbine components, for example blades or nozzles, can occur by various processes, such as by investment casting. In investment casting of relatively complex airfoil parts, intentional defects, such as xe2x80x9cbumper holes,xe2x80x9d may be required for casting the part, as is known in the art. The bumper holes constitute an xe2x80x9cintentionalxe2x80x9d defect used to hold the casting core during casting of relatively complex articles.
Ceramic bumpers are added to the ceramic core to limit the maximum distortion or motion of the core relative to the mold, to achieve a control of wall thickness in a cavity. The bumper holds the casting core in place during casting. After the core is removed, a thin spot remains where a bumper was located. This thin region is removed forming a xe2x80x9cbumper holexe2x80x9d that can then be repaired to achieve the full required wall thickness.
The bumper holes should be repaired when the casting is completed and prevent coolant leakage and to make the casting usable. Accordingly, post-processing of the investment casting is needed to remove the bumper holes.
Several proposed repair methods for cracks in components have been proposed. For example, European patent application EP 0740976 (EP 976) discloses a method of repairing single crystal metallic articles using a laser technique. EP 976 attempts to overcome problems associated with the laser weld repair of these articles by optimizing laser parameters. In particular, EP 976 provides a molten material at the crack, solidifies the molten material, and provides a re-melt of a once solidified melt from a second energy source, in an attempt to provide an acceptable stress-free repair. However, EP 976 does not discuss a repair of as-cast articles. Further, EP 976 does not provide for removal of defects, and does not provide for melting of the casting to insure a sound metallurgical bond and physical repair. Furthermore, the second application of energy in EP 976 is costly and inefficient with respect to both in time and power consumption.
It is desirable to reduce overall costs involved with casting. This cost reduction includes avoiding scrapping newly cast articles with manufacturing defects. This cost reduction also includes efficiently repairing, rather than scrapping and re-casting, parts with defects resulting from use of the part.
Accordingly, it is desirable to provide a method for repairing defects in airfoils that reduces costs associated with the production of airfoils.
Further it is desirable to provide a method for repairing defects in airfoil components and parts, which are produced by investment casting processes, that reduces costs associated with the production of said airfoil components.
It is also desirable to provide a process with means to repair defects, both intentional, such as xe2x80x9cbumper holexe2x80x9d defects, or unintentional, such as freckles and inclusions from the casting process or cracks resulting from use, thereby minimizing the need to scrap and recast. A reduction in process costs results in a savings to the manufacturer, and ultimately to the customer.
Therefore, it is desirable to provide a method of repairing defects in cast articles, where the defect comprises at least one of a manufacturing, intentional, or service-induced defect. The cast article comprises a casting core and a casting, the casting core comprising a ceramic bumper that creates a thin region in the casting comprising the defect. The method of repairing the defect comprises locating a defect at a defect area in the cast article; removing an area of the casting at the defect area; removing an area of the casting core including the bumper at the defect area where removing the area of the casting at the defect area and removing an area of the casting core including the bumper at the defect area creates a hole through a wall of the casting; positioning repair material in the hole; heating the defect area so the repair material and the area of the defect become molten; and resolidifying the molten material to form a repaired casting.
Also, it is desirable to provide a method, similar to that above, but without forming the hole, to repair surface defects, such as voids, freckles and inclusions.
Further, it is desirable to provide a repaired article formed by the methods, as embodied by the invention.
These and other aspects, advantages and salient features of the invention will become apparent from the following detailed description, which, when taken in conjunction with the annexed drawings, disclose embodiments of the invention.